In recent years, with an advantage of being capable of a super-high-speed and high-frequency operation, various electronic devices using a group III-V compound semiconductor mainly represented by GaAs have been dramatically developed by being used in high-frequency equipment such as a mobile phone, a satellite broadcasting receiver, and the like, and have since continued to make a steady progress in the development.
In general, in order to produce an electronic device using a compound semiconductor, there is used a semiconductor substrate obtained by laminating crystal layers having required characteristics by various methods such as an ion implantation method, a diffusion method, and an epitaxial growth method on a single crystal substrate. Among the methods described above, the epitaxial growth method has been widely used in the production of the semiconductor substrate of this type since it is possible to control not only the amount of an impurity but also a composition and a thickness of a crystal in an extremely wide range with high precision.
As the epitaxial growth method, a liquid phase method, a vapor phase method, and a molecular beam epitaxy method, which is a type of vacuum deposition methods, are well known. Among them, the vapor phase method is industrially widely used since it can process a large number of substrates with excellent controllability. In particular, a metal-organic chemical vapor deposition method (hereinafter referred to as MOCVD method) wherein a crystal growth is performed by initiating thermal decomposition over a substrate using an organic metal compound or a hydride of atomic species constituting an epitaxial layer as a material has a wide range of applicable substances, is suitable for the precise control of the composition and thickness of the crystal, and is excellent in mass productivity so that it has been widely used in recent years.
An epitaxial growth substrate for use in the production of electronic devices such as the FET, the HEMT, and the like is produced by growing a crystal layer of GaAs, AlGaAs, InGaAs, or the like each having required electronic characteristics on a GaAs substrate with a required structure by using, e.g., the MOCVD method.
In a planar electronic device such as the FET, the HEMT, or the like, there is grown an active layer which activates transistor characteristics by controlling electrons which laterally travel in a channel layer formed of a GaAs layer or an InGaAs layer using an electric field resulting from a gate electrode, and a buffer layer composed of the GaAs layer, an AlGaAs layer or the like is typically grown between the active layer and a semi-insulating substrate.
The object of inserting the buffer layer between the active layer and the semi-insulating substrate is to suppress the influence of an impurity at the interface between the epitaxial layer and the substrate, the influence of the substrate itself, and electron leakage from the active layer, and the buffer layer has a highly important role to retain the characteristics of the electronic device.
In the case where various epitaxial layers are grown on the substrate by the MOCVD method, since a group III material such as Ga, Al, or the like is supplied as the organic metal compound, it is known that carbon (C) is included into the grown crystal when the material is thermally decomposed to be grown as the epitaxial layer. Further, it is known that a thermal decomposition behavior of the organic metal compound is changed and the concentration of C to be included in the grown crystal is changed as well by a so-called group V/group III material flow rate ratio, which is a ratio between flow rates of a group III material such as gallium (Ga), aluminum (Al), or the like and a group V material such as arsenic (As), phosphorus (P), or the like. When the epitaxial layer of GaAs, AlGaAs, or the like is grown, as the epitaxial growth is performed by setting the group V/group III material flow rate ratio to a smaller value, the obtained epitaxial layer has a higher C concentration. Since C behaves as an acceptor impurity in the GaAs crystal or the AlGaAs crystal, the obtained epitaxial layer is a crystal layer having a p-type carrier density as a background concentration.
When the compound semiconductor epitaxial substrate for use in the production of the planar electronic device having the channel layer in which electrons travel is produced by the MOCVD method, a crystal layer having a background p-type carrier density such as a Schottky layer, a spacer layer, or the like is placed on the front side (the side opposite to a template) of the channel layer, while a crystal layer having the background p-type carrier density such as the spacer layer, the buffer layer, or the like is placed on the back side (the same side as the template) of the channel layer.
Therefore, when an epitaxial growth substrate for the electronic device such as the FET, the HEMT, or the like is produced by the MOCVD method, a plurality of crystals having the background p-type carrier density are grown.
In the production of the epitaxial substrate for use in the production of the electronic device such as the FET, the HEMT, or the like, in the case of, e.g., the structure of a pseudomorphic-HEMT (hereinafter referred to as p-HEMT) which is a HEMT using an InGaAs strained layer as the channel layer in which electrons travel, the mobility of electrons in the channel layer at room temperature (300 K) is about 8250 cm2/Vs (The institute of Electronics, Information and Communication Engineers, Proceedings of the IEICE General Conference 2006, CT-1-3 “Epitaxial growth technology for compound semiconductor microwave devices”, 25, Mar. 2006, Kokushikan University), and it has been difficult to achieve a value higher than the value mentioned above. Accordingly, there have been limitations on achieving reductions in transient resistance and power loss of the electronic device, and increasing the levels of characteristics of the electronic device to levels higher than the present levels by increasing the electron mobility.
Conventionally, various proposals have been made for an improvement in the electron mobility. For example, it is proposed in JP-A-06-21106 that the electron mobility is improved by optimizing the In composition of the InGaAs strained layer used as the channel layer and the thickness of the InGaAs layer by using a given relational expression in the p-HEMT structure. In addition, there is proposed in JP-A-02-246344 that a two-dimensional electron gas concentration and the electron mobility are improved by inserting a spacer layer made of the AlGaAs layer and the GaAs layer between the InGaAs strained layer used as the channel layer and an n-AlGaAs electron supply layer to optimize growth conditions in the p-HEMT structure. Further, there is proposed in JP-A-2004-207471 that the electron mobility is improved by combining an increase in the In composition of the InGaAs strained layer and the spacer layer made of the AlGaAs layer and the GaAs layer to optimize growth conditions.
However, from the viewpoint that the characteristics of the electronic device are improved as the two-dimensional electron gas concentration and the electron mobility have higher values in the compound semiconductor epitaxial substrate having the channel layer in which electrons travel such as the epitaxial substrate having the p-HEMT structure, the respective values of the two-dimensional electron gas concentration and the electron mobility are not satisfactory in the field to which devices for high frequency or the like are applied to, and therefore an epitaxial substrate having improved electron mobility characteristics with high two-dimensional electron gas concentration and high electron mobility has been in demand.